The present invention realtes to a compressor, and more particularly, to a variable displacement swash plate type compressor suitable for use as a refrigerant compressor for automobile air conditioners.
In conventional variable displacement swash plate type compressors which are disclosed in, for example, U.S. Pat. No. 4,428,718, the swash plate is constituted by a swash plate portion and a boss portion integrally formed with the swash plate portion for rotatably supporting a piston support. The swash plate is supported at its boss portion in such a manner that it is inclinable by a sole sleeve which slides along a driving shaft. In the above-described structure, the swash plate portion is offset from the center of inclination of the swash plate with respect to the sleeve.
Furthermore, the piston support is prevented from rotating together with the swash plate by restricting the movement of a support pin protruding from the outer periphery of the piston support in the direction of rotation of the shaft by means of a guide groove provided in a front cover.
Furthermore, a pivot pin fixed to the lug of the swash plate is restricted by a cocoon-shaped cam groove formed in the drive lug fixed to the shaft so as to form a line contact portion through which a thrust compressive force acting on the pistons is transmitted to a thrust bearing supporting portion of the shaft.
The conventional method of defining the minimum capacity of a variable displacement compressor for use in automobile air conditioners has been described from page E-45 to page E-47 in Section of Air Conditioners E6 of Service Weekly Report R32 published by Nissan Automobile Co., Ltd. in May, 1989.
In this method, when the capacity of the compressor is changed, the movement of a hinge ball along a drive shaft is restricted. That is, when the capacity of the compressor is reduced, the hinge ball slides along the drive shaft toward the piston, and is brought into contact with a retaining ring through a stroking spring, by which the minimum capacity is defined. Also, a thrust bearing is provided at the end portion of the drive shaft located closer to the piston.
In the aforementioned conventional compressor disclosed in U.S. Pat. No. 4,428,718, the thrust compressive force acting on the pistons acts on the line contact portion between the pivot pin and the cam groove in the drive lug, and generates a large amount of area pressure, causing wear.
Furthermore, in the above conventional technique, the inclination moment on the swash plate surface due to a centrifugal force generated by the rotational motion of the swash plate portion is small as compared with the inclination moment on the swash plate surface in the opposite direction due to the inertial couple generated by the reciprocating motion of the pistons, allowing unbalanced inclination moment due to the force of inertia to exist.
The unbalanced inclination moment due to the force of inertia increases in proportion to the square of the rotational speed of the driving shaft and makes reduction in the swash plate angle at a high-speed rotation difficult because the inclination moment is the moment acting in a direction in which the swash plate angle increases.
Furthermore, as stated above, since the swash plate portion is offset from the center of the rotation of the swash plate with respect to the sleeve, the center of gravity of the swash plate portion is offset from the central axis of the rotation of the driving shaft in accordance with the inclination angle of the swash plate with respect to the driving shaft, and the resultant force of the centrifugal forces on the individual portions of the swash plate does not become zero. These unbalanced centrifugal forces and the above-described unbalanced moment can be a force against the external members of the compressor and can be a cause of vibrations.
Particularly, since the magnitude of the unbalanced force of inertia changes as the inclination angle of the swash plate changes as a consequence of control of the capacity of the compressor, even if a balance mass is fixed to the driving shaft, it is impossible to balance the force of inertia at all the inclination angles.
Furthermore, in the conventional technique, the support pin protruding from the piston support and restricted by the guide groove in the front cover reciprocates in the guide groove through the slide balls and a shoe. The force of inertia generated in the axial direction at that time can also be a force to the external members and can be cause of vibrations. Also, sliding of the shoe against the guide groove may generate wear or seizure.
In the conventional compressor described in Service Weekly Report, when the capacity of the compressor is controlled such that it is reduced, the thrust load (or thrust), directed from the drive hub to the piston, acts on the drive shaft, thus necessitating provision of a thrust bearing for receiving this thrust load by the end portion of the drive shaft located closer to the piston.